Humans rapidly lost much of their sense of smell as they evolved to place a heavier emphasis on their sense of sight, according to a recent genetics study.

Although they have the same number of genes for smell detection as other primates (about 1,000), in humans more than half these no longer function, scientists reported in the journal, Proceedings of the National Academy of Science.

That decline took place within an 'evolutionary moment' of just three to five million years, and it happened four times faster in the branch of the evolutionary tree leading to humans than it did for other primates, said the team's leader, Professor Doron Lancet of the Weizmann Institute of Science in Israel.

Lancet's team earlier discovered that more than half of the genes that code for olfactory receptors in humans carry a mutation that prevents them from working properly.

In the latest study, they compared DNA sequences of 50 olfactory receptor (OR) genes that are common to humans and different species of apes and monkeys. Not all OR genes function in all species, so the proportion of working genes determines the sharpness of smell.

The researchers found that 54% of the genes were in humans were impaired 'pseudo-genes', as opposed to only 28% to 36% in the other species. Their study also made it possible to reconstruct the decline over the course of evolution, which suggested strongly that the sudden drop in the sharpness of smell was peculiar to Homo sapiens.

That loss probably stemmed from the specialised development of the human brain, which entailed greater emphasis on vision, the ability to distinguish colours and the capacity to identify individuals by facial appearance rather than by smell, the researchers suggested.

"We found that humans have accumulated mutations that disrupt OR coding regions roughly four-fold faster than any other species sampled," the authors said. "As a consequence, the fraction of OR pseudo-genes in humans is almost twice as high as in the non-human primates, suggesting a human-specific process of OR gene disruption, likely due to a reduced chemosensory dependence relative to apes."

Olfactory receptors located in the mucous lining of the nose identify scents by binding to molecules of odorous substances. Humans have about 12 million cells high up in the nasal cavity that carry olfactory receptors, whereas bloodhounds - specially bred for tracking by scent - have about four billion such cells.

Some people, however, have a much sharper sense of smell than others, a difference which may have a genetic basis. Only about one-fifth of OR genes do not function in mice, while in chimpanzees, gorillas, orangutans and rhesus macaques the proportion is close to one-third.

Lancet's research group has specialised in studying the OR genes, learning that they are the largest gene 'superfamily' in mammals. All human chromosomes - except chromosome 20 and the male Y-chromosome - carry OR genes, mostly grouped in clusters.

One group - known as 'fish like' and previously thought to be relics in more highly evolved four-limbed animals - were found to make up as much as 10% of the human repertoire, all of them in one large cluster on chromosome 11. The fact that relatively few of those are impaired suggests that they still have a functional significance.